The production process for glycols as described for instance in Kirk-Othmer, Encyclopedia of Chemical Technology, 4th Edition, Volume 12, pages 700 to 725, is a widely applied process. In such a process, monoethylene glycol or monopropylene glycol along with di- and triethylene glycol, or di- and tripropylene glycol (all of which are further referred to as glycols) are obtained by direct hydrolysis of the corresponding alkylene oxides. The process has the disadvantage of requiring a large excess of water for the reaction with alkylene oxides. Due to this excess the process consumes much energy. Furthermore, the process also has a limited efficiency due to the significant amount of undesired side products formed. A different process for production of glycols from alkylene oxides, which requires much less water and therefore has a lower energy requirement and a higher efficiency, is the reaction of alkylene oxide with carbon dioxide to obtain the corresponding alkylene carbonate, and the subsequent catalytic hydrolysis reaction with water or catalytic transesterification with alcohol to obtain the corresponding glycol.
The present invention pertains to the removal of organic halogen-containing compounds from streams containing one or more of glycol, water and alcohol.
Such streams can be produced by a process involving the steps of (a) reacting an alkylene oxide with carbon dioxide in the presence of a halide-containing catalyst to obtain a reaction mixture containing the corresponding alkylene carbonate, and (b) contacting the reaction mixture obtained in step (a) with water and/or alcohol in the presence of a suitable catalyst to obtain a reaction mixture containing glycol.
A disadvantage of the use of halide-containing catalysts in step (a) is that their presence also leads to formation of organic halogen-containing compounds.
U.S. Pat. No. 5,405,977 describes the removal of halogen-containing compounds from alkylene carbonates produced according to step (a) with the help of a halide-containing catalyst by contacting the contaminated alkylene carbonate with hydrotalcite under non-aqueous conditions. In the process described in U.S. Pat. No. 5,405,977, alkylene carbonate is prepared and then separated from the initial reaction mixture, for instance by one or more vacuum distillation steps prior to removal of the halogen-containing compounds.
Although the process described in this document represents a viable way to purify alkylene carbonates contaminated by halogen-containing compounds, there are several disadvantages associated with the process.
The required non-aqueous conditions make the process itself cumbersome to perform on an industrial scale, as all components need to be essentially water-free.
Furthermore, under conditions suitable for distillation of alkylene carbonates, part of the alkylene carbonate decomposes to alkylene oxide under loss of carbon dioxide, thereby reducing the overall yield, as indicated for instance in U.S. Pat. No. 5,510,499.
Moreover, alkylene oxide formed in this decomposition can react with residual halide-containing catalyst, thereby leading to an increased contamination of the alkylene carbonates with halogen-containing compounds. A further disadvantage resides in the fact that by treatment of alkylene carbonate with hydrotalcite, minute amounts of the corresponding alkylene glycol is produced. This alkylene glycol will however form an azeotropic mixture with the alkylene carbonate, and thus complicate the distillation, thereby leading to an increased decomposition of alkylene carbonate under the conditions of the distillation, as mentioned in U.S. Pat. No. 5,510,499.
In particular when the alkylene carbonate is further reacted with water and/or alcohol for the formation of glycol as in step (b) described above, an initial separation of alkylene carbonate from the reaction mixture would complicate the process, and reduce the overall yield of alkylene glycol.
On the other hand, in the subject process, i.e. when the product of step (a) is further subjected to step (b) as defined above, it would be expected that any halogen-containing organic compounds would be removed by hydrolysis or transesterification under the conditions usually applied. Such conditions comprise contacting the product of step (a) with water or alcohol in the presence of heterogeneous catalysts, such as stabilized magnesium oxides as for instance described in JP-A-06238165.
However, contrary to this expectation, it was found that the treatment usually applied in step (b) described above in the presence of alkylene carbonate and residual halide-containing catalyst led to an increase of the amount of organic halogen-containing compounds in the reaction mixture during the conversion of the alkylene carbonate, and did not lead to a sufficient reduction of these compounds even when continuing the reaction after the complete conversion of alkylene carbonate, at least under the conditions usually applied to avoid loss of alkylene carbonate to side reactions.
It would therefore be desirable to have a process for the preparation of alkylene glycol from alkylene carbonate, which avoids removal of the residual halide-containing catalyst at stages where this can affect the overall yield of alkylene glycol. It would likewise be desirable to be able to remove organic halogen-containing compounds without the requirement for non-aqueous conditions.